Lactoperoxidase (LPO) and myeloperoxidase (MPO) have been identified previously in human milk. These peroxidases have antimicrobial activity and presumably contribute to the protective functions of milk. In this study, we amplified genes encoding LPO and MPO from human mammary gland cDNA by the polymerase chain reaction (PCR). These genes were expressed in a baculovirus-insect cell system. Peroxidase activity was observed in the culture supernatant of Tricoplusia ni cells infected with the recombinant viruses and the levels increased upon addition of delta-aminolevulinic acid. Purified recombinant human LPO and MPO, both with a molecular mass of about 80 kDa, showed properties similar to bovine LPO and human MPG, respectively, in terms of absorption spectrum, sensitivity to dapsone, specificity for chloride ions, and reactivity with anti-bovine LPO or anti-MPO antibodies. Our data suggest that this expression system is useful for studying the catalytic mechanism and biological significance of these human peroxidases.